This invention relates to a projection exposure apparatus and a device manufacturing method using the same. For example, the invention is suitably applicable to a projection exposure apparatus or a scan type exposure apparatus to be used in a lithographic process, among device manufacturing processes for production of semiconductor devices such as IC or LSI, image pickup devices such as CCD, display devices such as a liquid crystal panel, or magnetic heads, for example, particularly in relation to projection of a pattern of a first object such as a reticle onto a second object such as a wafer through a projection optical system.
As regards a microprocessing technology for semiconductor devices such as IC or LSI, many proposals have been made on a reduction projection exposure apparatus (stepper) or a scan type projection exposure apparatus, for forming an image of a circuit pattern of a mask or reticle upon a photosensitive substrate through a projection optical system (projection lens) and for exposing the photosensitive substrate in accordance with a step-and-repeat method or a step-and-scan method.
In these exposure apparatuses, a pattern of a reticle must be transferred onto a wafer accurately in accordance with a predetermined magnification (reduction ratio). To this end, it is important to use a projection lens (projection optical system) having a good performance and small aberrations. Particularly, in order to meet recent requirements of further miniaturization of a semiconductor device, in many cases a pattern which is beyond the normal imaging performance of a projection optical system has to be transferred to a wafer. Thus, the aberration of a projection optical system becomes very influential to the pattern to be transferred. On the other hand, for the projection lens, enlargement of an exposure area as well as enlargement of its numerical aperture (NA) are desired, which are not convenient for aberration correction.
In these circumstances, it is desired to perform measurement of the imaging performance of a projection lens, particularly, wavefront aberration thereof, in a state that the projection lens is being mounted on an exposure apparatus, that is, a state that it is used for a practical exposure process.
An example of measurement methods for wavefront aberration of a projection lens is a phase restoration method. This method has been used in the field of electron microscopes or astronomical telescopes having large aberrations, for improvement of the resolution. In accordance with this phase restoration method, a phase distribution of an image is detected on the basis of image intensity distributions at plural positions such as image plane, pupil plane, and defocus position, for example. From the detected phase distribution, a wavefront aberration of an optical system is calculated.
In this phase restoration method, an actually measured intensity distribution of an image on an image plane is used and, after an arbitrary phase is applied, Fourier transform is made thereto to detect a complex amplitude distribution upon a pupil plane. Subsequently, while keeping a phase component of the thus detected complex amplitude distribution, only an absolute value corresponding to an intensity component thereof is replaced by a value (root square of the intensity at the pupil plane) corresponding to the actually measured value. The result is then taken as a fresh complex amplitude distribution, and inverse Fourier transform is made thereto, to determine a complex amplitude distribution upon an image plane. Again, while keeping its phase component, the intensity is replaced by an actually measured value.
By repeating the above-described calculations, complex amplitude distributions on the image plane and the pupil plane are calculated and, from the phase distribution of the complex amplitude distribution at the pupil plane, the wavefront aberration of the projection lens is detected. The phase restoration method will be described later in more detail, in conjunction with preferred embodiments of the present invention.
Where a wavefront aberration of a projection lens is to be calculated in accordance with the phase restoration method, idealistically it is necessary to measure an intensity distribution of an image under a condition of coherent illumination ("sgr"=0). If the value "sgr" (that is, a ratio of the numerical aperture of an illumination-system to the numerical aperture of the projection lens) becomes larger, the calculated wavefront aberration contains a larger error. For example, if the wavefront aberration should be calculated with a precision of about 0.01xcex, a relation "sgr"xe2x89xa60.1 is required. Even though the precision is lowered to about 0.03xcex, a relation "sgr"xe2x89xa60.2 has to be satisfied. On the other hand, when a pattern of a reticle is to be photo printed on a wafer, usually the reticle is illuminated under a partially coherent illumination condition. Thus, normally, an illumination system of an exposure apparatus has "sgr" which is in a range of about 0.2 less than "sgr" less than 0.9. No illumination system as providing "sgr"xe2x89xa60.2 is loaded. Further, many illumination systems for an exposure apparatus are equipped with an incoherency-transforming mechanism.
For these reasons, when a wavefront aberration of a projection lens is to be detected in accordance with the phase restoration method while using an illumination optical system for a practical exposure process as it is, there is a problem with respect to the precision.
It is an object of the present invention to provide a projection exposure apparatus and/or a device manufacturing method using the same, by which a wavefront aberration of a projection optical system (projection lens) for projecting a pattern of a mask onto a wafer can be measured very precisely and by which production of a large integration device can be facilitated.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.